In order to preserve efficiency, DC-DC converters have to adapt to different load conditions. Therefore, many DC-DC converters are usually configured to distinguish light-load and high-load conditions. In a buck converter for example, under high-load conditions, the DC-DC converter typically operates in a continuous operation mode. Under high load conditions in continuous operation mode, the inductor current typically does not reverse. Under light-load conditions, DC-DC converters typically enter a “discontinuous” operation mode. In discontinuous operation mode, the DC-DC converter becomes only active when the output voltage falls below a certain threshold voltage level. The operation during this discontinuous mode is to recharge the output capacitor ideally without the inductor current reversing. DC-DC converters are often designed to change from continuous to discontinuous mode when load current reduces sufficiently to result in the reversal of the inductor current.
Step down DC-DC converters typically have an energizing switch (the high-side switch) and commutating switch (the low-side switch) which are coupled at a switching node. The function of the energizing switch is to energize the inductor from the power source. The function of the commutating switch is to commutate the inductor current when not being energized by the energizing switch. The switching node is coupled to the inductor. The two switches are driven by two non-overlapping clock signals having the same clock period. For a step-up converter, a reverse current through the inductor can be detected if both switches are turned off (non-conducting) while the voltage level at the switching node is sensed. However, in continuous operation mode, the energizing switch and the commutating switch are only turned off simultaneously during a short period of time which is only a fraction of the time of the clock period. Therefore, prior art solutions typically need an additional high speed comparator to detect the light-load condition, i.e. the reversal of inductor current. High speed comparators are rather complex and consume a substantial amount of power, in particular if the clock frequency for the DC-DC converters is rather high.